The present invention relates generally to aluminum alloys and more particularly to high-strength, molybdenum-containing, powder-derived aluminum alloys which have improved resistance to environmentally-assisted cracking at room or moderately elevated temperatures.
High-strength aluminum alloys, such as those in the 7000 series (Aluminum Association designation), are used extensively for structural aircraft components because of their combined lightness of weight and high strength. Generally speaking, these high-strength aluminum alloys suffer from poor resistance to environmentally-assisted cracking. For instance, when under load and subjected to a corrosive environment such as the sea water environment, these alloys are susceptible to stress corrosion cracking. The corrosive elements attack the aluminum surface, causing it to develop pitted areas, which serve as a potential nucleus for cracks. Further corrosion occurs within these cracks, which, in conjunction with the load, causes them to propagate, possibly resulting in catastrophic failure. Similarly, corrosion fatigue, or failure of the metal from corrosion combined with cyclic stress, is a problem with these high-strength aluminum alloys.
Various approaches have been tried to improve the stress corrosion cracking response of these high-strength aluminum alloys. New heat treating methods, such as overaging, and retrogression and re-aging, have been tried, and although these methods are effective in improving the alloy's stress corrosion cracking response, the alloy's mechanical properties decline. Surface coatings containing molybdates have been applied to aluminum, which react with the aluminum's protective oxide film to inhibit the development of surface pits. Once a surface pit does develop, however, the coating does little to retard the growth of cracks emanating therefrom.
Powder metallurgy and rapid solidification processing techniques have been used to produce aluminum alloys having superior properties to alloys produced by conventional ingot metallurgy. For instance, molybdenum has been added to aluminum using these techniques to give the aluminum high strength at high temperatures. When molybdenum-containing aluminum is exposed to high temperature, molybdenum compounds such as molybdenum aluminide form, providing the added strength. Similarly, when molybdenum is added to aluminum as a melt ("prealloying"), molybdenum aluminide is formed upon solidification. No improvement in stress corrosion cracking resistance is observed in these high-temperature aluminum alloys over aluminum alloys not containing molybdenum.